1. Field of the Invention
The present invention relates to a label sheet for electrophotography and an image forming method with use thereof, used for electrophotographic copying machines and printers.
2. Description of the Related Art
Accompanied by colorization and digitization of electrophotographic copying machines and printers, higher resolution and speed-up in electrophotography have been studied. Particularly, digitization of inputting and outputting images has been developed for full-color electrophotographic copying machines and printers in order to respond to higher resolution and higher processing speed of images, and thereby methods for inputting images and methods for processing, developing, transferring and fixing input images have been significantly improved. Further, each component, developers, photo-conductors, and the like, has also been improved corresponding to high definition and high quality of color recording in digital mechanisms.
Accompanied by higher resolution of full-color copying machines and printers, A variety of media such as not only high quality paper but also coated paper, cast-coated paper, plastic materials, seal materials and the like having large basis weight have been used for recording media. Particularly, seal and label materials and the like have been used for labels and a variety of usages such as photo seals have been proposed. The materials of such seals and labels have been developed using high quality paper, coated paper, plastic or the like as a base (base member).
However, in cases of using a recording medium composed of a high quality paper base, there is a tendency for heat-melted toner to be loaded into the high quality paper at the time of fixing, which creates a rough touch and non-uniformity of image gloss on the image. Owing to this, images having high quality cannot be obtained. In cases of using a recording medium composed of a coated paper base at high humidity, there is a tendency for the recording medium to produce blisters when heated while fixing. Owing to this, the recording medium cannot be used. In cases of using a recording medium composed of a film base, the stiffness of the recording medium becomes poorer by heating while fixing. Owing to this, there is a tendency for the recording media to be wound around the fuser (often referred to as xe2x80x9cfuser windingxe2x80x9d hereinafter) and to produce marks of winding. Further, the process tends to produce curls curving in an arc in a portion or whole of the recording medium by thermal shrinkage of the recording medium. Owing to this, when the curls are significant, the edge portion of the recording medium comes to be folded in the machine (edge fold).
Full-color copying machines and printers have been developed to respond to speed-up in addition to higher resolution, which causes the recording medium a large heating load in the heat-fixing device, resulting in a lot of troubles on the fixing device described above.
In order to improve the problems concerning the curls on the fuser described above, in Japanese Patent Application Laid-Open (JP-A) No. 2000-235275, Japanese Patent No. 1941236 and Patent No. 2807062, it is disclosed that curls are improved by defining the thermal shrinkage rate of the whole film medium (label sheet for electrophotography). However, the label sheet (label sheet for electrophotography) comprises a surface base member (label body) and a back surface base member (separator), both of which are temporarily adhered via an adhesive (adhesive layer). The surface base member and the back surface base member are different from each other in forms, structures and the like. Owing to these differences, when the difference in the thermal shrinkage between these members is large, large curls which induce a trouble on the copying machine or printer is produced, resulting in that the techniques described in the publications described above cannot solve the problem about the curls of the label sheet.
On the other hand, in JP-A No. 11-300898, the technique defining thermal shrinkage rate of a label body base member (label base member) is disclosed. However, it is difficult for this technique to resolve the problem about curls when the shrinkage rate of a separator is large.
To improve these problems, the technique defining shrinkage rates of the surface and the back surface (label body and separator) of a label sheet to 1% or less respectively is disclosed in JP-A No.06-337537. However, even though each shrinkage rate of the surface and back surface is within 1%, if difference in the thermal shrinkage rates between those of the surface and the back surface is large, it is difficult for the technique described in the Publication to suppress appearance of curls, which may cause problems about curls.
In cases of using a resin film as a recording medium, since stiffness of the film becomes poor significantly by heating upon fixing, it may often occur that the resin film is wound around the fuser or marks of winding are produced on the resin film. Further, it is not possible to make image gloss of printed matters printed using a resin film uniform. On the other hand, in cases of a label sheet using high quality paper as a base member of a label body and/or a separator, curls may occur by significant expansion and shrinkage of the label sheet caused by the change of temperature and humidity, and the image printed on the label body expands with the label sheet and cracks because the image cannot go along with the expansion (such cracks is referred to as xe2x80x9ctoner crackxe2x80x9d hereinafter).
Further, to improve fuser winding, the technique defining the elasticity and stiffness of a recording medium at an ambient temperature is disclosed in JP-A Nos. 62-116945 and 11-231671. However, the elasticity and stiffness of the recording medium remarkably decrease by heating on fixing. For this reason, it is not possible to suppress the fuser winding even though the elasticity and stiffness of the recording medium at an ambient temperature are defined.
On the other hand, the technique that an image receiving layer (toner image receiving layer) with a thickness of 4 xcexcm or more made of a thermoplastic resin having 50 to 80xc2x0 C. of glass transfer temperature is provided on the label body of a label sheet and that the image gloss is made uniform by loading toner on the image receiving layer into the image receiving layer when fixing is disclosed in JP-A No. 7-248636.
With the technique described in this Publication, image gloss can be uniformed by provision of an image receiving layer. However, curls may easily occur after fixing because it is not particularly taken into consideration to prevent occurrence of curls in that Publication. Moreover, the label sheet easily produces winding around the fuser and marks of winding because the image receiving layer does not have releasability.
From the above description, when printing is done using a label sheet fabricated by the technique in the Publication described above, it is not possible to prevent from producing winding of a label sheet around the fuser and marks of winding on the label sheet when it is printed. Even though such winding of a label sheet around the fuser is not produced, it is difficult to prevent problems such as appearance of curls of printed matters, edge folds due to remarkable curl appearance, toner cracks of printed matters by changes of temperature and humidity and the like.
The present invention has an object to solve the problem described above. In other words, an object of the present invention is to provide a label sheet for electrophotography composed of a resin film base and an image forming method with use thereof. The label sheet of the present invention can prevent problems such as label sheet winding around a fuser and marks of winding produced on printing. Moreover, it prevents other problems such as appearance of curls of printed matters, edge folds caused by appearance of significant curls, toner cracks of printed matters generated by changes of temperature and humidity and the like.
The above object is achieved by the present invention described hereinafter. According to a first aspect of the present invention, there is provided a label sheet for electrophotography, the label sheet comprising: a label body including a label base member and an adhesive layer disposed on one surface of the label base member, the label base member including a resin film; and a separator including at least a separator base member, the separator base member including a resin film, the separator being releasably bonded to the adhesive layer, and the separator including a stiffness B, as determined by the following formula (2), of at least 0.21 Nxc2x7mm:
B=E t3xe2x80x83xe2x80x83Formula (2) 
in which E represents tensile elasticity in N/mm2 of the separator at 135xc2x0 C., and t represents thickness in mm of the separator at 23xc2x0 C., 50% relative humidity, wherein the separator further includes a thermal shrinkage rate in an MD direction thereof, when subjected to heat treatment at 150xc2x0 C. for 5 minutes, of S % and the label body further includes a thermal shrinkage rate in an MD direction thereof, when subjected to heat treatment at 150xc2x0 C. for 5 minutes, of L %, such that a thermal shrinkage difference A as determined by the following formula (1):
A=Sxe2x88x92Lxe2x80x83xe2x80x83Formula (1) 
is in the range xe2x88x920.3% to 0.1%.
In the label sheet for electrophotography, a toner image receiving layer containing a thermoplastic resin having a storage elastic modulus in the range of 4 Pa to 250 Pa at 130xc2x0 C. may be formed on a surface of the label base member opposite to the surface provided with the adhesive layer.
Further, the label sheet for electrophotography may comprises the toner image receiving layer which contains a releasing agent in the range of 1 weight % to 10 weight %.
According to a second aspect of the present invention, there is provided a method for forming an image, the method comprising the steps of: providing a label sheet for electrophotography, the label sheet including a label body which includes a label base member and an adhesive layer disposed on one surface of the label base member, and a separator which includes at least a separator base member, the label base member including a resin film, the separator base member including a resin film, the separator being releasably bonded to the adhesive layer, and the separator including a stiffness B, as determined by the following formula (2), of at least 0.21 Nxc2x7mm:
B=Exc2x7t3xe2x80x83xe2x80x83Formula (2) 
in which E represents tensile elasticity in N/mm2 of the separator at 135xc2x0 C., and t represents thickness in mm of the separator at 23xc2x0 C., 50% relative humidity; forming a toner image with toner on the label body of the label sheet; and fixing the toner image to the label body of the label sheet, including melting the toner by heat, wherein the separator has a thermal shrinkage rate in an MD direction thereof, when subjected to heat treatment at 150xc2x0 C. for 5 minutes, of S % and the label body has a thermal shrinkage rate in an MD direction thereof, when subjected to heat treatment at 150xc2x0 C. for 5 minutes, of L %, such that a thermal shrinkage difference A as determined by the following formula (1):
A=Sxe2x88x92Lxe2x80x83xe2x80x83Formula (1) 
is in the range xe2x88x920.3% to 0.1%.
The fixing step may be carried out using a belt-type fixing device.
Furthermore, the fixing step may be a step where fixing treatment is conducted two times and the second fixing treatment is performed by using a belt-type fixing device.